The use of printing cylinders comprising a mandrel and a printing sleeve are known from the art, for example, for flexographic or (lithographic) off-set printing. Generally, the mandrel is an air mandrel that comprises a rigid cylindrical body, such as a steel shaft, on which a printing sleeve is removably mounted. The cylindrical surface of the air mandrel contains outflow openings via which air can be supplied when a sleeve has to be mounted on the mandrel or removed from the mandrel. A variety of printing sleeves with different radii can be mounted to provide the printing cylinder diameter required for a printing job. The image to be printed is provided directly on the printing sleeve or may be provided on a (flexible) printing plate or mold that is mounted on the printing sleeve using techniques known from the art.
Mounting the printing sleeve on the mandrel is often performed using compressed air. The printing sleeve is a substantially cylindrical body having a through hole with an inner diameter that is slightly smaller than the outer diameter of the mandrel. This allows the printing sleeve to fit with a press fit or interference fit on the mandrel. The printing sleeve further comprises at least one radially deformable or radially compressible layer that enables an inner surface of the printing sleeve to expand radially outwardly under pressure, for example by using compressed air. The printing sleeve is positioned in line with the mandrel, after which compressed air is supplied via the outflow openings provided in the cylindrical outer surface of the mandrel. The compressed air causes a radially outward expansion of the inner surface of the printing sleeve, therewith increasing its inner diameter. The increase in inner diameter is sufficient to slide the printing sleeve over the mandrel. Upon ending the supply of compressed air, the printing sleeve inner surface shrinks to provide the interference fit or press fit between the inner cylindrical surface of the sleeve and the outer cylindrical surface of the mandrel.
The precision of the known printing sleeves should be improved to obtain a more consistent and accurate printing result. The precision of a printing cylinder or printing sleeve can be indicated by a parameter called the total indicated run out value or TIR-value. The higher the TIR-value, the lower the precision. In fact, the TIR-value is an indication of the margin within which the outer cylinder surface may extend around the theoretically desired diameter of the outer surface. In other words, the TIR-value is an indication of the tolerance which is defined by the difference between the minimum and maximum diameter around a theoretically desired diameter. The smaller this margin, the smaller the TIR-value and the better the precision of the printing cylinder or printing sleeve. Part of the problems of the imperfect precision of the known printing sleeves is caused by the radially compressible layer that is present in the printing sleeve and that is needed to be able to mount the known sleeve on the air mandrel. As a result, the outer surface of printing sleeve may displace with respect to the central axis of the air mandrel. This causes a deviation which is reflected by an increase of the TIR-value of the printing cylinder. As explained above, higher TIR-value corresponds with a larger deformation and, as a result, with a lower print quality. Another disadvantage of the sleeves having an compressible inner layer is that such sleeves have a limited life time in view of the deterioration of the compressible inner layer.
US 2014/0311368 discloses an air-mountable printing sleeve for mounting on a mandrel, wherein the printing sleeve is a multi-layered cylindrical sleeve provided with at least two rigid radial spacer members that substantially replace the deformable layers. The printing sleeve comprises an inner layer and an outer layer that are connected by two rigid, circular spacer members disposed at the opposite extreme ends of the printing sleeve. The inner layer comprises a deformable material that is radially expandable or radially deformable. The inner surface of the inner layer has an inner diameter that is slightly smaller than the outer diameter of a mandrel, which inner diameter can be increased using for example compressed air. This allows the printing sleeve to be mounted on the mandrel with an interference fit. The outer layer of the printing sleeve is made of a material that is rigid and non-expandable by compressed air. The outer layer is fixedly connected with at least two rigid spacer members comprising annular rings that extend radially and circumferentially in an empty space between the inner layer and the outer layer. The outer annular surface of each extreme end of the inner layer is fixedly connected to the inner annular surface of a corresponding end spacer member. The end spacer members connect the inner layer and the outer layer. Any spacer members not disposed on the extreme outer ends of the printing sleeve are separated from the inner layer with a gap between the inner surface of the spacer members and the outer surface of the inner layer. The gap is very small, for example in the order of fractions of a millimeter. The gap allows the expansion and shrinkage of the inner layer required for mounting the printing sleeve to the mandrel.
A disadvantage of the printing sleeve according to US 2014/0311368 is that the gap between the spacer members and the inner layer of the printing sleeve allow vibration and deformation of the printing sleeve, thus reducing the TIR-value and the print quality. Furthermore, the inner layer and the outer layer of the printing sleeve are only connected to each other on the opposite extreme ends of the printing sleeve, which is a disadvantage, especially in printing sleeves with a greater length.
This problem has been recognized in WO2006114534 of which the US-equivalent is US2009031910 and which represents the closest prior art. This publication discloses a printing shaft assembly on which a metal printing sleeve can be mounted. The connection between the metal printing sleeve and the shaft assembly is effected by metal washers of which the radially outer ends are inclined relative to a plane that extends perpendicular to the axis of the shaft assembly. By virtue of a clamping force exerted in the axial direction on the metal washers, the radial outer ends are deformed so that the washers become more flat and obtain an increased outer diameter. The outer circumferential edge of the radially outer ends thus engages the inner surface of the printing sleeve and performs a clamping action. The publication discloses two sets of washers disposed at the two axial extremities of the shaft assembly and the sleeve cooperating therewith. The contact surface between the washers and the sleeve is very small and compressing the washers requires a complicated control assembly including a control shaft that is axially moveably arranged in a support shaft of the shaft assembly, transmission rings and cotter pins that pass through the transmission rings, the support shaft and the control shaft. The transmission rings and cotter pins are provided adjacent both axial extremities of the support shaft and the control shaft extends over the entire length of the support shaft through the support shaft. This complicated control assembly is necessary for compressing the two sets of washers to the substantially same extend when clamping between the support shaft and the printing sleeve is needed. Consequently, the shaft assembly known from WO2006114534 and US2009031910 is beneficial in that it provides the possibility to use an exchangeable metal printing sleeve. However, the clamping force that may be obtained with the axially compressible washers is limited and the construction for the compression of the washers is complicated.